1. Field of the Invention
The present invention relates generally to the field of optical communications. More specifically, the present invention discloses a system for redirecting optical communications in response to a fault in an optical link.
2. Statement of the Problem
Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information over optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, for example, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into individual channels or sets of channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels to output ports and permutes these channels according to control signals to create a desired coupling between an input channel and an output port.
Fiber optic communications networks are typically arranged with a plurality of terminals in any of a number of topological configurations. The simplest configuration is two terminals communicating data over an optical link. This can be extended to a daisy-chain configuration in which three or more terminals are connected in series by a plurality of optical links. Ring configurations are also used, as well as other two-dimensional networks. In each case, the optical link between two terminals typically includes a plurality of optical fibers for bidirectional communications, to provide redundancy in the event of a fault in one or more of the optical fibers, and for future capacity.
A fault in an optical fiber can be detected by the receiving terminal by sensing a loss of signal or severe degradation of signal power. Lesser defects in an optical link can be detected by monitoring the error rate in the received data. In any case, the conventional approach in responding to such faults has been to reroute the entire WDM signal from the faulty optical fiber to one of the unused optical fibers. This may also require rerouting the WDM signal through one or more intermediate nodes in the optical network. In ring configurations, one approach to responding to a fault has been to redirect communications in the opposite direction around the ring to avoid the faulty segment of the ring.
Another approach has been to reroute the WDM signal from the faulty optical fiber to a second optical fiber that has degree of unused capacity. For example, the interrupted signal can be translated to an unused frequency band and transmitted over a second optical fiber using a frequency-division multiplexing scheme. Similarly, the interrupted signal can be assigned to unused time slots and transmitted over a second optical fiber using a time-division multiplexing scheme. All of these prior art approaches require significant overhead to dynamically allocate unused fibers, frequencies, and/or time slots, and then reroute the signals to the appropriate fibers.
3. Other Related Art
The Applicants' U.S. Pat. Nos. 5,724,165 and 5,694,233, and U.S. patent application Ser. No. 08/739,424 teach two methods for high performance signal routing (U.S. Pat. No. 5,724,165) and wavelength demultiplexing (Ser. No. 08/739,424 and U.S. Pat. No. 5,694,233). In U.S. Pat. No. 5,724,165, new structures for realizing optical switches (routers) were disclosed that achieve very high extinction ratios. However, these switches are wavelength independent. In Ser. No. 08/739,424 and U.S. Pat. No. 5,694,233, optical systems are disclosed to provide the functions of wavelength demultiplexing and routing.
Other related art in the field includes the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Fee 5,777,761 July 7, 1998 Fee 5,731,887 Mar. 24, 1998 Ogura 5,517,489 May 14, 1996 Glance 5,488,500 Jan. 20, 1996 Shiragaki 5,457,556 Oct. 10, 1995 Yamane 5,434,691 July 18, 1995 Patel et al. 5,414,541 May 9, 1995 Goosen et al. 5,396,357 Mar. 7, 1995 Meadows 5,381,250 Jan. 10, 1995 DeJule et al. 5,363,228 Nov. 9, 1994 Iino et al. 5,343,464 Aug. 30, 1994 Flanagan et al. 5,159,595 Oct. 27, 1992 Cozic 5,081,452 Jan. 14, 1992 Nelson 4,919,522 Apr. 24, 1990 ______________________________________
Ammann, "Synthesis of Electro-Optic Shutters having a Prescribed Transmission vs Voltage Characteristic," Journal of the Optical Society of America, vol. 56, no. 8, pp. 1081-1088 (August 1966)
Harris et al., "Optical Network Synthesis Using Birefringent Crystals I. Synthesis of Lossless Networks of Equal-Length Crystals," Journal of the Optical Society of America, vol. 54, no. 10, pp. 1267-1279 (Oct. 1964)
U.S. Pat. Nos. 5,777,761 and 5,731,887 (Fee) disclose an optical communications system in which a controller reroutes the signal through a protection link in the event of link failure. If necessary, the controller can reroute the signal through a frequency translator.
Ogura discloses a bidirectional optical ring in which channels (i.e., time slots) are dynamically allocated.
Glance discloses a tunable add/drop filter using a 1.times.N optical switch, a wavelength grating router (WGR), and a multiplexer. The WGR outputs include a set of retain outputs that are coupled directly to the multiplexer and a drop output. The particular WDM frequency component that is routed to the drop output is determined by the WGR input port at which the WDM signal is received. The 1.times.N switch provides the WDM signal to the proper WGR input so that a selected frequency is provided to the drop output. The retained signals and any added signals are multiplexed by the multiplexer.
Shiragaki discloses an optical cross-connect system using space and wavelength division switching to remedy faults in a optical communications network.
Yamane discloses an example of an optical communications system that uses time multiplexing for protection.
Patel et al. disclose an optical switch using a series of birefringent layers and ferro-electric cells to route an input beam to any of a plurality of output positions.
Goosen et al. disclose a ring configuration with a single optical fiber in which optical signals of two discrete wavelengths are transmitted in opposite directions over the ring. This ensures that each of the nodes on the ring will receive identical data irrespective of any single point failure in the ring.
Meadows discloses a 2.times.2 electro-optical switch that employs dielectric film polarizing beamsplitters and a switchable electro-optic retarder.
DeJule et al. disclose an optical switching device using a plurality of polarization-independent switching cells arranged in matrix form. Each switching cell consists of a spatial light modulator and a number of polarized beamsplitters that can be used to selectively direct an input optical beam along either of two axes.
Iino et al. disclose a switching system that includes a protection switch that operates to switch a working transmission line to a protection transmission line using time multiplexing.
Flanagan et al. and Cozic disclose ring communications systems in which signals are rerouted in the opposite direction in the event of a fault.
Nelson discloses an optical switch employing an electro-optical crystal that exhibits birefringence in each of two different light paths when the crystal is disposed in orthogonally-oriented electric fields. Each light path is sensitive to a different one of the two electric fields and has its own set of fast and slow axes.
Ammann and Harris et al. provide general background in the field of optical filter design.
4. Solution to the Problem
None of the references discussed above show the present system for automatically multiplexing and redirecting WDM signals in the event of a fault in the optical link between two terminals. Under normal operating conditions in the present invention, the WDM signals are transmitted as two separate, mutually exclusive sets of channels over two optical fibers between the terminals. For example, the two sets of channels can be interdigitally spaced. If a fault is detected in one optical fiber, the transmitting terminal combines both sets of channels and routes the combined channels over a second optical fiber that does not have a fault. The receiving terminal demultiplexes the combined channels to recreate the first and second sets of channels for further transmission over additional links in the optical network.